PROJECT 1 Abstract Despite increasing recognition of tick as a highly efficient disease vector, our knowledge of its immune system, especially how the vector recognizes and mounts antimicrobial responses to invading pathogens has been very limited. Recently, we discovered a novel cross-species signaling cascade that allows Ixodes scapularis tick to sense a specific mammalian factor present in the infected blood meal as an infection cue, triggering a potent microbicidal response that limits the proliferation of invading pathogens like Borrelia burgdorferi. These studies also uncovered a critical role of a tick ?immune? GTPase, the Ixodes GTPase (IGTPase) in the tick gut, that is induced by ingested mammalian IFN?. The IFN?, through downstream Ixodes STAT signaling events, activates IGTPase to generate potent borreliocidal responses. In the current Project 1 of our Program Project Grant (PPG), we propose to investigate the molecular details of how this interferon-like defense operates in ticks. In our first series of experiments, we will systematically define how IFN? induces the tick JAK/STAT pathway, including its regulation of IGTPase. We will investigate how IGTPase-induced tick antimicrobial proteins limit B. burgdorferi persistence in the vector. Besides, recent studies showed that arthropod immunity is influenced by crosstalk between discrete signaling pathways, which may impact the persistence of pathogens with overlapping range, such as B. burgdorferi and A. phagocytophilum. Therefore, our second group of experiments will examine how IFN?-like tick defense system impacts the persistence of additional pathogens like Anaplasma phagocytophilum. We will also define the crosstalk between immune pathways, which will synergize the studies proposed in other projects of our PPG - Project 2 for defining mechanistic operation of the immune deficiency pathway (IMD) with other tick immune pathways, and the gut microbiota which is the focus of Project 3. These mechanisms altogether ultimately govern the persistence of B. burgdorferi and A. phagocytophilum. Overall, our study will shed new insight into the operation of cross- species signaling mechanisms involving discrete or co-operative tick innate immune pathway(s) that influence persistence of multiple tick-borne pathogens, including co-infection in ticks.